The present invention relates to a thermocontrol method for an injection molding machine, more precisely relates to a method for controlling temperature of thermocontrolled components, e.g. an injection cylinder, of the injection molding machine to correspond to the instant operating status thereof.
Conventionally, PID (Proportional-Integral-Differential) control method is usually adopted to control the temperature of thermocontrolled components, e.g. an injection cylinder, of an injection molding machine.
The PID control method is based on a proportional action (P), which is in proportion to a control deviation; an integral action (I), which is based on an integrated value of the control deviation; and a differential action (D), which is based on differential coefficients of the thermocontrolled components.
The PID control is performed on the basis of a following numerical formula: ##EQU1## Y: control value t: time
.epsilon.: control deviation PA1 Kp: proportional sensivity PA1 Td: differential time PA1 Ti: integrated time
In the PID control, actional sensitivities of the P, I and D controls can be adjusted by changing the coefficients Kp, Td and Ti. Note that, the D control can be eliminated when the coefficient Td is zero; the I control can be eliminated when the coefficient Ti is infinitely great, so P control, PI control or PD control may be adopted if possible.
Using the PID control, temperature can be maintained when the thermocontrolled components are under certain stable conditions.
However, the injection molding machine has various operating states such as stop, temperature rise, mold, pause, etc.. Furthermore, heating elements and cooling elements are different devices in each status.
For example, in the temperature rise state of an injection cylinder, radiant heat is the main cooling element. On the other hand, heat from the heater, the heat of friction caused by an injection screw acting upon the resin, etc. are the heating elements in the mold status; while the natural radiant heat, the endothermic reaction by resin supplied, etc. are the cooling elements therein.
Therefore, in conventional PID control where each control value is fixed even if the status of the injection molding machine changes, the temperature of an injection cylinder, for example, may be higher (overshoot, P.sub.o) or may be lower (under shoot, P.sub.u) than the object temperature, as shown as a curve P in FIG. 5: A Temperature Graph of the Injection Cylinder. The overshoot P.sub.o and undershoot P.sub.u of the injection cylinder have a large effect on the viscosity of molten resin, so that they may become factors in inferior production quality. Moreover, the overshoot P.sub.o causes resin deterioration when the object temperature is exceeded and the resin deterioration temperature is attained, resulting in inferior products.
When precise control is required, to avoid inferior production quality, manual control of the temperature of the injection cylinder, based on the experience of a skilled operator, is required because automatic precise control cannot be performed by conventional PID control.
On the other hand, to eliminate overshoot and the undershoot for thermocontrol of the injection cylinder, etc. by the PID control, the PID control should be always performed corresponding to the status of the injection molding machine.